What is vitamin B12 deficiency anemia?

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What is Vitamin B12 Deficiency Anemia

Vitamin B12 deficiency anemia is a type of macrocytic anemia caused by inadequate vitamin B12 (cobalamin), resulting in impaired DNA synthesis that leads to ineffective red blood cell production, characterized by large, immature red blood cells (megaloblasts) and often accompanied by neurological complications. 1

Pathophysiology

Vitamin B12 is essential for DNA synthesis, methylation, and mitochondrial metabolism 2. When deficient, cells cannot properly synthesize DNA, leading to:

  • Ineffective erythropoiesis: The bone marrow produces abnormally large, immature red blood cells (megaloblasts) with nuclear-cytoplasmic asynchrony 3
  • Intramedullary destruction: These defective cells are destroyed within the bone marrow before entering circulation, causing elevated lactate dehydrogenase, indirect hyperbilirubinemia, and reduced haptoglobin—mimicking hemolytic anemia 3
  • Shortened red cell lifespan: The abnormal cells that do enter circulation have reduced survival 3

Clinical Manifestations

Hematological Features

The classic presentation includes 1, 3:

  • Macrocytic anemia with mean corpuscular volume (MCV) typically >100 fL
  • Peripheral blood smear showing oval macrocytes, anisopoikilocytosis, and hypersegmented neutrophils (>5 lobes)
  • Pancytopenia in severe cases—anemia accompanied by leukopenia and thrombocytopenia
  • Inappropriately low reticulocyte count for the degree of anemia

Non-Hematological Manifestations

B12 deficiency extends beyond blood abnormalities 1:

Neurological symptoms:

  • Peripheral neuropathy: pins and needles, numbness (paresthesia)
  • Myelopathy (spinal cord disease): balance issues, impaired gait, falls from impaired proprioception
  • Optic nerve dysfunction: blurred vision, visual field loss, optic atrophy

Cognitive symptoms:

  • "Brain fog," difficulty concentrating, short-term memory loss

Other features:

  • Glossitis (inflamed tongue)
  • Unexplained fatigue
  • Hyperpigmentation of knuckles and oral mucosa 3

Risk Factors and Causes

The 2024 NICE guideline identifies key risk factors 1:

Dietary insufficiency:

  • Vegan or vegetarian diets without B12 supplementation
  • Religious dietary restrictions excluding animal products
  • Food insecurity or inability to prepare B12-rich foods

Malabsorption:

  • Atrophic gastritis affecting the gastric body
  • Pernicious anemia (autoimmune destruction of parietal cells)
  • Celiac disease
  • Gastric or small intestine resections

Medications:

  • Metformin (especially >4 months use)
  • Proton pump inhibitors or H2 receptor antagonists (>12 months)
  • Colchicine, phenobarbital, pregabalin, primidone

Other conditions:

  • Autoimmune diseases (thyroid disease, Sjögren syndrome, type 1 diabetes)
  • Family history of B12 deficiency

Diagnostic Approach

First-line testing when B12 deficiency is suspected 1:

  • Either active B12 (holotranscobalamin) or total B12 (serum cobalamin) is acceptable
  • Active B12 is more accurate (measures biologically active form) but more expensive (£18 vs £2) and has longer turnaround time
  • Complete blood count to assess for macrocytic anemia

Confirmatory testing 1:

  • Methylmalonic acid (MMA) when initial B12 results are in the indeterminate range—provides more reliable functional assessment of B12 status
  • Recent data suggests total B12 ≤257 ng/L (Roche assay) likely reflects severe deficiency, while 258-341 ng/L may indicate early depletion requiring homocysteine confirmation 4

Clinical Significance

Prevalence: Subclinical deficiency affects 2.5-26% of the general population depending on definition used 2. Clinical deficiency with overt manifestations is less common but carries serious consequences if untreated.

Severe presentations occur in approximately 10% of cases and can mimic hematological malignancies, aplastic anemia, or thrombotic microangiopathies 3. In pediatric patients, particularly infants of B12-deficient mothers, presentations may include profound anemia, pancytopenia, failure to thrive, and recurrent infections 3.

Reversibility: The critical point is that B12 supplementation leads to rapid and often complete hematological recovery 3. Hemoglobin levels can rise from 9.7 g/dL to 12.6 g/dL within six weeks of treatment, with MCV normalizing and reticulocyte counts appropriately increasing 5. Functional improvements in gait, strength, and activities of daily living are also documented, particularly in older adults 6.

Important Caveats

  • Neurological damage can occur even without anemia in some cases
  • Neurological symptoms may be irreversible if treatment is delayed, making early recognition crucial
  • The condition is often underdiagnosed because symptoms are non-specific and develop gradually
  • High B12 levels (>350-1,200 pmol/L) paradoxically associate with malignancy and hepatic disorders, requiring different clinical consideration 7

References

2
Research

Vitamin B12 deficiency.

Nature reviews. Disease primers, 2017

7
Research

Vitamin B12 deficiency - A 21st century perspective .

Clinical medicine (London, England), 2015

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